1. Field of the Invention
The present invention relates to a heat-sensitive, negative working lithographic printing plate precursor.
2. Description of the Related Art
Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. In addition to the well known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, heat-sensitive printing plate precursors have also become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization, or by particle coagulation of a thermoplastic polymer latex.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder, e.g., a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in, e.g., EP-A 625,728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer latex, as described in, e.g., EP-A 770 494, EP-A 770 495, EP-A 770 496, and EP-A 770 497. These patents disclose a method for making a lithographic printing plate including the steps of (1) image-wise exposing a plate precursor having a heat-sensitive image-recording layer to infrared light, wherein the image-recording layer includes hydrophobic thermoplastic polymer particles, sometimes also referred to as latex particles, which are dispersed in a hydrophilic binder, and (2) developing the image-wise exposed element by applying water or by mounting the plate on the plate cylinder of a press and then supplying fountain solution and/or ink. During the development step, the unexposed areas of the image-recording layer are removed from the support, whereas the latex particles in the exposed areas have coalesced to form a hydrophobic phase which is not removed in the development step. In EP-A 1 342 568, a similar plate precursor is developed with a gum solution and in EP 1614538, EP 1614539, and EP 1614540, development is achieved by means of an alkaline solution.
EP 1 356 926 discloses a negative-working lithographic printing plate precursor including on a grained and anodized aluminum support having a surface roughness expressed as arithmetical mean center-line roughness Ra less than 0.45 mm, a heat-sensitive coating including hydrophobic thermoplastic polymer particles.
A lithographic printing plate precursor including on a hydrophilic support an image-forming layer containing a polymerizable compound, an initiator, and a compound containing a functional group having an interaction with the surface of the hydrophilic support is disclosed in EP 1 500 498.
EP 1 155 820 discloses a printing plate including on a support an undercoating layer, a first layer containing a polymer soluble in an alkaline aqueous solution, and a second layer containing a cyanine dye as an infrared absorbing agent and a cross-linking or polymeric compound which forms a covalent bond by action of light and/or heat and thereby lowers solubility of the second layer in an alkaline developing liquid.
EP 1 106 381 discloses a printing plate precursor including on a grained and anodized aluminum substrate having an average roughness Ra of 0.5 mm or less and/or micropores having a pore diameter of 1 to 5 nm and a pore density of 8×1015 to 2×10/m2, a photosensitive layer containing an infrared absorbing agent and a polymer soluble in an aqueous alkaline solution whose solubility in the solution varies by infrared laser exposure.
WO 2003/010006 and WO 2004/066029 disclose a printing plate precursor including on a hydrophilic lithographic base a coating including uncoalesced particles of a hydrophobic thermoplastic polymer and a non-crosslinkable aqueous-soluble composition which is capable of facilitating the removal of the unexposed portions of the coating in an aqueous developing solution. Examples of non-crosslinkable aqueous-soluble compositions include inorganic salts, organic bases, organic acids, and/or metal complexes.
A problem associated with plate precursors that work according to the mechanism of heat-induced latex coalescence is that after the coating step the surface of the printing plate precursor may show so-called point defects. These point defects may be visible throughout the surface of the coating and have varying dimensions; they are believed to be caused by local coagulation of latex particles. After exposure of such plate precursors, the image areas as well as the non-image areas may contain these point defects. During development, these spots are not always completely removed and may result in toning at the non-image areas (ink acceptance in the non-image areas). Especially, high sensitivity plates which include latex particles that are only weakly stabilized and thus coalesce readily, i.e., upon exposure at a low energy density, tend to show this problem of point defects.